Paper products are used for a variety of purposes. Paper towels, facial tissues, toilet tissues, and the like are in constant use in modern industrialized societies. The large demand for such paper products has created a demand for improved versions of the products. If the paper products such as paper towels, facial tissues, toilet tissues, and the like are to perform their intended tasks and to find wide acceptance, they must possess certain physical characteristics. Among the more important of these characteristics are absorbency, softness, and strength.
Absorbency is the characteristic of the paper that allows the paper to take up and retain fluids, particularly water and aqueous solutions and suspensions. Important not only is the absolute quantity of fluid a given amount of paper will hold, but also the rate at which the paper will absorb the fluid. Softness is the pleasing tactile sensation consumers perceive when they use the paper for its intended purposes. Strength is the ability of a paper web to retain its physical integrity during use.
There is a well-established relationship between strength and density of the web. Therefore efforts have been made to produce highly densified paper webs. One of such methods is disclosed in the U.S. Pat. No. 4,112,586 issued Sep. 12, 1978; the U.S. Pat. Nos. 4,506,456 and 4,506,457 both issued Mar. 26, 1985; U.S. Pat. No. 4,899,461 issued Feb. 13, 1990; U.S. Pat. No. 4,932,139 issued Jun. 12, 1990; U.S. Pat. No. 5,594,997 issued Jan. 21, 1997, all foregoing patents issued to Lehtinen; and U.S. Pat. No. 4,622,758 issued Nov. 18, 1986 to Lehtinen et al.; U.S. Pat. No. 4,958,444 issued Sep. 25, 1990 to Rautakorpi et al. All the foregoing patents are assigned to Valmet Corporation of Finland and incorporated by reference herein.
This technology uses a pair of moving endless bands to dry the web which is pressed and moves between and in parallel with the bands. The bands have different temperatures. A thermal gradient drives water from the relatively hot side, and the water condenses into a fabric on the relatively cold side. While the web is wet and under pressure and elevated temperature, a combination of temperature, pressure, moisture content of the web, and residence time causes the hemicelluloses and lignin contained in the papermaking fibers of the web to soften and flow, thereby interconnecting and "welding" the papermaking fibers together.
While the described technology allows production of a highly-densified strong paper suitable for packaging needs, this method is not adequate to produce a strong and--at the same time--soft paper suitable for such consumer-disposable products as facial tissue, paper towel, napkins, toilet tissue, and the like. It is well known in the art that increasing the density of a paper generally decreases the paper's absorbency and softness characteristics, which are important for the consumer-disposable product mentioned above.
Cellulosic structures currently made by the present assignee contain multiple micro-regions defined most typically by differences in density. The differential density cellulosic structures are created by--first, an application of vacuum pressure to the wet web associated with a molding belt, thereby deflecting a portion of the papermaking fibers to generate low-density micro-regions, and--second, pressing portions of the web comprising non-deflected papermaking fibers against a hard surface, such as a surface of a Yankee dryer drum, to form high-density micro-regions. The high-density micro-regions of the resulting cellulosic structure generate strength, while the low-density micro-regions contribute softness, bulk and absorbency.
Such differential density cellulosic structures may be produced using through-air drying papermaking belts comprising a reinforcing structure and a resinous framework, which belts are described in commonly assigned U.S. Pat. No. 4,514,345 issued to Johnson et al. on Apr. 30, 1985; U.S. Pat. No. 4,528,239 issued to Trokhan on Jul. 9, 1985; U.S. Pat. No. 4,529,480 issued to Trokhan on Jul. 16, 1985; U.S. Pat. No. 4,637,859 issued to Trokhan on Jan. 20, 1987; U.S. Pat. No. 5,334,289 issued to Trokhan et al on Aug. 2, 1994. The foregoing patents are incorporated herein by reference.
As well known in the papermaking art, wood typically used in papermaking inherently comprises cellulose (about 45%), hemicelluloses (about 25-35%), lignin (about 21-25%) and extractives (about 2-8%). G. A. Smook, Handbook for Pulp & Paper Technologists, TAPPI, 4th printing, 1987, pages 6-7, which book is incorporated by reference herein. Hemicelluloses are polymers of hexoses (glucose, mannose, and galactose) and pentoses (xylose and arabinose). Id., at 5. Lignin is an amorphous, highly polymerized substance which comprises an outer layer of a fiber. Id., at 6. Extractives are a variety of diverse substances present in native fibers, such as resin acids, fatty acids, turpenoid compounds, and alcohols. Id. As used herein, hemicelluloses, lignin, and polymeric extractives inherently present in cellulosic fibers are defined by a generic term "fluid-latent indigenous polymers" or "FLIP." Hemicelluloses, lignin, and polymeric extractives are typically a part of cellulosic fibers, but may be added independently to a plurality of papermaking cellulosic fibers, or web, as part of a papermaking process.
Traditional papermaking conditions, such as the temperature of the web and duration of the application of pressure during transfer of the moist web to the Yankee dryer, are not adequate to cause FLIP to soften and flow in the high-density micro-regions.
The commonly assigned co-pending patent applications entitled "Differential Density Cellulosic Structure and Process for Making Same" filed on Jun. 6, 1997 and "Fibrous Structure and Process for Making Same" filed on Aug. 15, 1997, both of which are incorporated by reference herein, disclose the process for making cellulosic and fibrous structures comprising micro-regions formed by a process of softening the fluid-latent indigenous polymers inherently contained in and/or added to the cellulosic papermaking fibers, then allowing the fluid-latent indigenous polymers to flow thereby interconnecting the adjacent papermaking fibers of the high-density micro-regions, and finally immobilizing the fluid-latent indigenous polymers in the high-density micro-regions. In order to achieve sufficient fluidization of the fluid-latent indigenous polymers contained in the web, the web must be subjected to an intensive heating for a certain period of time (a residence time). Reduction of the residence time can provide significant increase in the speed of the papermaking process and, consequently, a sufficient economic benefit.
U.S. Pat. No. 4,729,175, issued to Beard et al. on Mar. 8, 1988, discloses a method and apparatus for applying ultrasonic energy to a continuously moving web of paperboard, while simultaneously press-drying and heating the web. Now, it is believed that a suitable field of ultrasonic energy can be coupled to the web in order to initiate fluidization of the fluid-latent indigenous polymers contained in the web. Additionally or alternatively, the application of the ultrasonic energy enhances the fluidization of the fluid-latent indigenous polymers, if the ultrasonic energy is applied to the web while the web is heated. It is believed that the ultrasonic vibrations coupled to the web assist in fluidization of the fluid-latent indigenous polymers due to internal absorption of the ultrasonic energy by the fluid-latent indigenous polymers and their shear thinning, i.e., decrease of the viscosity of the fluid-latent indigenous polymers. The use of ultrasonic energy can, therefore, help to reduce the residence time necessary to achieve the fluidization of the fluid-latent indigenous polymers and thus create conditions for speeding up the entire papermaking process.
Accordingly, it is the purpose of the present invention to provide an improved papermaking process comprising a step of ultrasonically assisted softening of the fluid-latent indigenous polymers contained in the web.
It is another object of the present invention to provide an improved papermaking process in which the heating energy produced by a conventional heating means and the ultrasonic energy produced by an ultrasonic means are coupled together to work in concert to accelerate fluidization of the fluid-latent indigenous polymers contained in the web.
It is another object of the present invention to provide an improved papermaking process for making a cellulosic structure having a plurality of high-density micro-regions and a plurality of low-density micro-regions, the plurality of high-density micro-regions comprising bonds of the fluid-latent indigenous polymers contained in the cellulosic web.
It is still another object of the present invention to provide an apparatus for the process of making a cellulosic structure having a plurality of high-density micro-regions comprising bonds of the fluid-latent indigenous polymers, the apparatus having an ultrasonic means for contributing to the formation of the bonds.